Powdery Mildew (Oidium, White Mold)
Powdery mildew (or simply PM) is one of the most widespread plant diseases across a variety of agricultural and horticultural crops, and one of the most common diseases cannabis cultivators face.
It may be caused by a variety of different fungi, though three in particular have been detected in cannabis: Podosphaera macularis, Golovinomyces cichoracearum and Leveillula taurica. While each species may prefer different environmental conditions and host genotypes, the signs and symptoms of infection are typically the same; chlorotic spotting, followed by white or grey, powdery spores generally form on the upper surface of the leaves and stems. These signs often present first on leaves exposed to the highest humidity and/or lowest air movement. You might also find powdery mildew on other aerial parts of the plant, such as the underside of leaves or flowers.
Powdery mildews may be even problematic in warm and dry climates, since some species do not require standing water to germinate but only brief windows of high humidity. They thrive in periods of environmental fluctuation, in areas with poor air circulation and/or crowded conditions.
How Does Powdery Mildew Spread?
Different species of powdery mildew have different temperature and humidity tolerances, and under favorable conditions they spread through the development of spores that travel from leaf to leaf and from plant to plant through the air column and water droplets. Powdery mildew is often introduced via infected cannabis clones, and spores can survive on clothing, gloves, or other equipment and surfaces for upwards of 36 hours. As a result, PM can be easily spread within a growth environment or between grow rooms.
Powdery Mildew vs Trichomes
While powdery mildew can colonize trichome laden areas on the plant as the infection progresses, it generally has a very different look as compared to the glistening effect produced by a heavy covering of trichomes. Powdery mildew typically has a less glistening and more opaque white flour-like appearance that often begins on leaves before spreading to other aerial parts of the plant.
Identifying Powdery Mildew
- Powdery and fuzzy flour-like white round patches on plant leaves, particularly those exposed to the high humidity and/or low air circulation.
- Distorted and yellowing leaves.
- Discolored plant tissues.
- Often happens in late veg and during flowering, though it can happen at any point in the life cycle when spores are present and environmental conditions favorable.
Powdery Mildew Treatment
While some species of PM common to cannabis do not systemically infect the whole plant, they can invade plant cells deeply enough that they can reappear after being treated. With more serious and extensive infections plants and perhaps even full crops may need to be destroyed.
For mild infections:
- Spray symptomatic and non-symptomatic plants with a contact-kill integrated pest management (IPM) product such as H202, ZeroTol, sodium bicarbonate or micronized sulfur to prevent spores from spreading during pruning and defoliation
- Remove infected leaf material and defoliate as needed
- Sterilize the growth environment with chlorine dioxide or other oxidizing agent
- Treat with biological fungicide
Treatments will vary depending on when the infection is detected. During vegetative growth, IPM products such as sulfur and biological fungicides can be applied without concern. Depending on the stage of flower, those same products will have a negative effect on the development and quality of the final flower.
Powdery Mildew Prevention
Because PM can be hard to completely eliminate, prevention is the best offensive against this common fungal infection.
- Sterilize the growth environment between planting rounds and after infections
- Use only pest- and pathogen-free clones or quarantine traditional clones before introducing the clones to the grow environment.
- Use air filters with a MERV rating of 12 or higher.
- Maintain proper ventilation and airflow. Do not crowd plants.
- Remove standing water and try to avoid splashing when working with water in the grow room. A shop vacuum equipped with a HEPA filter is a useful tool to have on hand. Be sure to empty the vessel when the shop vac is not in use.
- Ensure you can maintain stable and appropriate relative humidity levels
- Keep the temperature as stable as possible, with the room fluctuating no more than +/- 15°F.
- Use biological fungicides that contain Reynoutria sachalinensis weekly or biweekly to stimulate the plant’s natural defenses.
- Check regularly for signs of powdery mildew, particularly in late veg and during flowering
- Defoliate as needed.
- Appy contact kill products (H202, ZeroTol, micronized sulfur, sodium bicarbonate) regularly, followed by a biological fungicide during veg and into the first two weeks of the flowering cycle.
Botrytis (Bud Rot, Grey Mold)
Botrytis cinerea is a fungal infection that can strike at any phase of the plant’s life cycle, including post-harvest during drying and curing. Unlike PM, Botrytis attacks dead or dying plant tissue. Cultivars may have differing levels of tolerance to Botrytis infection, but it remains the most common humidity related disease in cannabis and can easily develop when humidity levels are high.
Given its apt nickname “bud rot,” this fungal infection often finds perfect conditions within the relatively dense structure and moist conditions of a maturing flower, where it targets dead or dying bracts and destroys the bud from the inside out. Botrytis thrives in environments between 50–80°F with high relative humidity and free water on plant or grow room surfaces.
Identifying Bud Rot
- Early signs include brown soggy spots on buds or brown spots/cankers on stems.
- More advanced signs present as a gray-brown mass of spores.
Bud Rot Treatment
- In mild infections, remove affected leaves using proper disposal and sanitation techniques to reduce further spread. Heavily infected plants should be destroyed.
- Avoid cross contamination to uninfected plants by implementing proper sanitation protocols.
- Sanitize all tools and equipment used.
- Use approved biological and botanical fungicides.
Bud Rot Prevention
- Employ robust integrated pest management (IPM) programs to avoid insect-induced mechanical injury to plant tissue, an easy entry point for infection.
- Remove standing water from the grow room and avoid free water on plant surfaces.
- Do not overwater.
- Prevent all instances of standing water or pooled water in the growing environment. A shop vacuum equipped with a HEPA filter is a useful tool to have on hand. Be sure to empty the vessel when the shop vac is not in use.
- Avoid drastic fluctuations in relative humidity within the plant canopy.
- Do not crowd plants.
- Provide proper ventilation.
Hop Latent Viroid
One of the stealthiest cannabis diseases is hop latent viroid (or HLVd). Also known as“dudding disease,” HLVd creates problems beyond visibly stunted growth in leaves and flowers. It is an infectious and stable RNA-based viroid that can lay dormant in plants for extended periods of time.
When the disease does rear its head, you may notice that once-healthy plants have become slow growing and stunted. HLVd will cause vegetative plants to remain short with smaller leaves, and it can reduce the quality and quantity of flower produced. Buds will be smaller and looser, with fewer trichomes, decreased potency, aroma and flavor.
HLVd poses a particular challenge since it can be hard to detect without specific testing and monitoring. Since it can go unnoticed even in symptomatic plants, HLVd is often spread through clones or inadequate horticultural hygiene. The only way to confirm its presence is through specific testing which can be done in-house or through a cannabis testing lab.
Studies have shown that viroids can remain infectious for longer than 24 hours on most common surfaces, 7 weeks in water, and months to years in dried plant debris and seeds. Careful and intentional sterilization is critical to limit HLVd’s spread.
Identifying Hop Latent Viroid
- Plants do not reach their full size potential. Growth is stunted.
- Small leaves with more horizontal branching.
- Downward curving or drooping branches.
- Branches become fragile and are easily broken.
- Overlapping leaflets.
- Small, loose buds with lower trichome production.
- Once plants are infected there is no way to treat or cure HLVd.
- Sterilize tools and grow environments with a bleach solution between grow cycles—isopropyl alcohol will not denature the viroid
- Use only pest- and pathogen-free clones or quarantine and test traditional clones before introduction to the grow environment
- Test crops and mothers for this viroid regularly
- Maintain tight sanitation protocols since HLVd can be easily spread through tools and touch.
- Do not crowd plants, as it can be directly spread from plant to plant, particularly after pruning or trimming
- Research is ongoing, but it’s suspected that insects may also be able to transmit the viroid through bites, so robust pest management may also help mitigate the spread.
Rice Root Aphids
Rice root aphids (Rhopalosiphum rufiabdominalis) are becoming increasingly common pests for cannabis cultivators across the US and Canada. These aphids can be found in both indoor and outdoor settings. As their name suggests, these particular aphids attack the plant’s root system, and once established they can cause significant damage if left untreated.
Unlike other aphids that cannabis cultivators might face, root aphids pose a particular challenge since most of their life cycle occurs below the medium’s surface. These aphids reproduce asexually with populations doubling every two days. Winged adults will eventually develop to spread the population.
Identifying Rice Root Aphids
These pests can be found in both soil and soilless mediums (including hydroponics). Look out for:
- Check roots closely during transplanting and under pots periodically for black/brown/dark green colored aphids on the surface of the roots
- Reduced vigor and slowed growth in the plant.
- Yellow and wilting leaves.
- Dark-bodied, winged adult aphids present at the medium’s surface.
Rice Root Aphid Treatment
- Introduce beneficial organisms such as rove beetles and predatory soil mites to slow the spread, but be aware they cannot completely eradicate the population once a colony is established.
- Most biological insecticides are not effective against rice root aphids due to their tough waxy coating.
- Pyrethrum is effective. However, the aphids cannot be completely eliminated unless the root ball is fully submerged.
- Use fine-mesh bags as a physical barrier around each pot to contain the spread.
Rice Root Aphid Prevention
- Periodically examine the medium’s surface for winged adults.
- Hang sticky traps.
- Use fine mesh bags around pots.
Tissue Culture Remediation for Cannabis Plant Diseases
Hard-to-treat pests and impossible-to-cure diseases cost the cannabis industry hundreds of millions of dollars in pest management and lost revenue each year, making prevention the best defense for any cultivation operation.
As a Tissue Culture leader for the cannabis industry, Conception is on the cutting edge of restoring juvenile vigor to prized genetics while offering guaranteed pest- and pathogen-free GenZero tissue culture clones. We work with the industry’s best breeders to offer a cultivar library that includes the most in-demand strains alongside award-winning classics—all clean, robust and ready for your next grow.
Learn more about the benefits of Tissue Culture for your business or check out some of the most popular options in our Tissue Culture Clone Library. Or, reach out to request the full menu of cultivars we provide and start working with disease-free genetics today.